I have no experience with foxes, so this is probably a bad choice for a simile. However, I don't need actual experience with something for a metaphorical to work – I just need to be able to imagine it. For example:

It was working as well as a car with a missing wheel.

I never saw that but I can imagine it.

So, returning to that first example, I will try to imagine a clever fox. Not the one I saw sleeping in the zoo; not the one I saw trotting across the road at night. But I read a children's book that had a clever fox, so I imagine that fox.

So, the simile seems to work – he is as clever as the fox I am imagining in my head. But I did all of the work, and fox accomplished nothing.

This is the illusion of a descriptive metaphorical. Readers try to imagine everything, including the metaphoricals. So they try to imagine the metaphor working, and when they do, it seems to describe. But that metaphor could be failing – a little or a lot – in actual description.

The handrail was as cold and wet as a toad's belly. (Farewell, My Lovelyy, Chandler)

I try to imagine that toad's belly. Um, that image is very concrete. And vivid. And creepy, so that simile is very moody. (The mood of creepy does not fit the scene, alas.)

What about accuracy? Toads are always described as having "dry" skin, but of course they could be wet, say if they were out in the rain. I imagined a cold wet toad because I was told to. So, the simile seems to work for description – it's saying the hand rail is cold and wet. But leaving out toad does that just as well.

The handrail was cold and wet.

This illusion is pervasive: When a metaphorical does not help description, it will almost always seem to be helpful, but only because the reader already knows enough to imagine the intended meaning.

It's also pernicious, and nondescriptive metaphors are epidemic. when you are editing, your metaphorical will seem to be working for you, because of this illusion. That doesn't mean it actually works.

So, if you want to get rid of your metaphoricals that aren't describing, you need to analyze your metaphorical. That's the biggest part of the third step in creating a good metaphorical – is this metaphorical actually working? This step requires critical examination, not naive confidence.